Circuit breakers are widely used in residential and industrial applications for the interruption of electrical current in power lines upon conditions of severe overcurrent caused by short circuits or by ground faults. Prior art devices have used current transformers (CTs) to sense overcurrent conditions: the output from the CT is used as input to an electronic trip breakers in which the signal indicating an overcurrent condition is used to trip the circuit breaker and interrupt the flow of electricity.
Electronic trip circuit breakers using CTs cannot sense DC current in power circuits for the reason that CTs only respond to AC current. At DC (zero) frequency, the output of a CT is zero so that a circuit incorporating a CT as a current measuring device has a 100% error. Even at frequencies of 30 Hz, prior art CT devices have a significant error in current measurement.
In order to be able to sense DC, devices using a Hall Effect element have been used. Hall effect devices require relatively large power sources, more amplification of their output signal and require the magnetic field to pass through the thin dimension of the chip rather than the length. Hall devices typically generate a signal of approximately 5 mV/100 gauss applied field and are limited to a temperature of less than 150.degree. C. Because of their relatively low sensitivity, magnetic fields of 100-400 gauss are needed. This means that their usage is limited to high current uses; or else, bulkier magnetic cores are required to concentrate the field. Other prior art methods have relied on monitoring the voltage drop across a resistive element to measure current. For multiphase currents, the signal to noise ratio of the resistive shunt is poor: the signal to be detected could be as low as a few millivolts on a background of several hundred volts.
The present invention is a method and apparatus that overcomes the drawbacks of prior art electronic trip circuit breakers and is capable of responding to AC and DC fault currents.